Circuit board with screening arrangement against electromagnetic interference

ABSTRACT

A circuit board (1) which comprises a plurality of electrically conductive layers, conductive patterns formed in the layers, and intermediate electrically insulating layers, and which further comprises a surface-related layer (3) which screens against electromagnetic interference, wherein an outermost conductive layer (3) is provided with a plurality of coordinated connection islands (3a-3e) or the like, where selected connection islands are adapted for electrical and mechanical coaction with connection pads (2a-2d) or the like for a surface-mounted discrete component (2). A further electrically conductive layer (4) disposed contiguous with and beneath the outermost conductive layer (3) and separated by an insulating layer is comprised of a layer foil that screens against electromagnetic interference. The electrically conductive layer foil has formed therein a plurality of through-penetrating holes (4a, 4e), each of which is adapted and dimensioned with a cross-sectional size which slightly exceeds the cross-sectional size of an electric conductor (2a) drawn through a respective hole (4a), the electric conductor (2a) being connected at one end, either directly or indirectly, to one of the connection pads (2a-2d) and at its other end directly to a conductor (6a) in a conductor pattern formed in an inner, further electrically conductive layer (6).

TECHNICAL FIELD

The present invention relates to a circuit board and more particularly,but not exclusively, to circuit boards of the kind that are comprised ofa plurality of superposed electrically conductive layers such as to formconductive patterns, and electrically insulating layers there between.

More particularly, the present invention is concerned with theconstruction of a circuit board that includes surface-relatedelectrically conductive layers which screen against electromagneticinterference, wherein one such outermost conductive layer is providedwith a plurality of coordinated connection islands where selectedislands are adapted for electrical and mechanical coaction withconnection conductors, connection pads, connection pins or the like or adiscrete surface-mounted component.

DESCRIPTION OF THE BACKGROUND ART

Several different designs of such circuit boards are known to the art.

The latest development within electronic equipment tends towardsincreasing signal speed and towards increasing the compactness ofdiscrete circuits on circuit boards. These measures has led to problemsrelating to electromagnetic interference (EMI).

Various measures have also been proposed in endeavours to reduce orfully eliminate the electromagnetic interferences that result primarilyfrom higher signal rates and short switch-over times.

Concerns other than EMI are related to electromagnetic compatibility(EMC) and electromagnetic discharge (EMD).

It is also known to fully "cover" or enclose circuits of this kind in anelectrically conductive material so as to obtain effective screening toand from the surroundings.

It is also known to fully encapsulate circuit boards in a metalliccasing and to provide delimited holes in the casing through which cablesand conductors can be connected to the circuit board.

In the case of circuit boards, it is known to attempt to "cover" andenclose the signal-carrying conductors formed in one or moreelectrically conductive layers in the form of copper foils inwardly of acircuit board having external earth-potential related electricallyconductive layers.

An example of this technique is given in publication EP-A2-0 238 267.

Such measures are taken in order to prevent external electromagneticinterference fields from obtaining access to the interior of the circuitboard and also to prevent the emission of electromagnetic fieldsgenerated by board components as interference fields.

It is also known in circuit boards to endeavour to utilize mutuallyopposing external electrically conductive layers and to connect saidlayers to earth potential such as to give said layers a screeningfunction.

However, such circuit boards normally require electric conductors to beincluded in these screening layers.

The provision of such conductors requires conductor-related foilsections to be isolated from the electrically conductive layer or fromthe remainder of the foil, wherein it is known to form said conductorsby etching away selected sections of the metal foil, such as to enablethe conductors to be formed without contact with the remainder of themetal foil.

In this regard, it is known to permit that part or those parts of themetal foil or the outer copper layers that have not been used asconductors for connection purposes and which have not been etched awayto remain on the circuit board and to connect this part or these partsto earth, so that the layer will obtain a partial screening function.

It is quite natural that screening layers etched in this manner areunable to provide a complete or a maximum screen, since severaldifferent conductor patterns have been formed therein with peripheralexposure of the metal foil.

Even though encapsulation forms part of the earlier standpoint oftechniques in preventing electromagnetic interference, this techniquewill not be described for the sake of simplicity.

The earlier standpoint of techniques also includes the realization thatscreening problems are heightened with increasing frequencies ofelectromagnetic fields and interference fields.

It has been discovered in the development of electronics towards stillhigher working frequencies and shorter switch-over times that ascreening technique of the aforesaid kind is no longer satisfactory.

One of the reasons for this is apparently because the conductors formedin the outer metal layer form radiation sources. Foil sections that havebeen etched away provide screen-free surfaces through which fields andinterference fields are able to pass.

In certain cases, these circumstances may result in a screening layeramplifying outwardly directed radiation and become highly receptive toexternal radiation or interference fields.

The reason for this is believed to be because chosen dimensions ofopenings provided in the copper layer for forming said conductors withinthe electrically conductive layer and the chosen lengths of theconductors can result in correspondence to an adapted antenna length ora part thereof for certain frequencies of functions used in the circuitboard.

By way of example, it can be mentioned that an oscillation of 3 GHz inair results in a wavelength of 10 cm, and a conductor length of 2.5 cm.generates an antenna function having a 1/4 wavelength.

It will be noted that a conductor structure having the aforesaid adaptedlength is reciprocal, meaning that it functions both in a transmittingand a receiving mode, which in the latter case lowers the immunity thata screen is intended to offer, and certain length dimensions will affordgood reception conditions for corresponding external interferencefrequencies.

With regard to a further standpoint of techniques in this regard,reference can be made to the disclosures made in U.S. Pat. No.5,341,274.

A bottom layer (4) is comprised of a layer produced from insulatingmaterial and can be readily formed with the aid of a stencil-printingprocess. Subsequent to having formed the bottom layer (4), a conductivelayer (5) is formed in a stencil-printing process within a region (12)located beneath an IC circuit. There is then applied a top layer (6) inthe form of an insulating material formed around the whole surface of asubstrate (1), with the exception of parts (2a) that are intended toreceive the pin connectors of an integrated circuit.

The upper layer (6) can be formed from the same material as the bottomlayer (4). The publication also discloses that the electricallyconductive layer (5) is formed so as to have no contact with theconnections (2a) of the integrated circuit in the region between the pinconnections, as shown in FIG. 8b.

The frequency components generated within this region will thereby belimited and radiation noise can be dampened.

U.S. Pat. No. 5,274,193 also teaches a multi-layer circuit board whichutilizes an electromagnetic screening layer produced from a treatedcopper paste and applied so as to cover substantially the whole surface.

Publication EP-A1-0 578 888 discloses that a circuit board may beprovided with an electrically conductive rail which extends alongmutually opposite sides and along a part of its rear side, therewithcreating the possibility of connecting the circuit board to an earthplane and therewith create conditions for screening against EMIradiation.

The subject matter of U.S. Pat. Nos. 5,323,299, 5,335,147 and 5,006,667also forms part of the prior art.

SUMMARY OF THE INVENTION Technical Problems

When considering the earlier standpoints of technique, as describedabove, it will be seen that a technical problem resides in enabling,with the use of simple means, a circuit board used at high ratedigitalized information-carrying signals and short switch-over times forused signal structures to be effectively screened againstelectromagnetic interference (EMI) by constructing a furtherelectrically conductive layer from a layer foil which screens againstelectromagnetic interference and which is located within the circuitboard.

It will also be seen that a technical problem resides in realizing thesignificance of and the advantages associated with simplifying theproduction of an effectively screened circuit board by locating afurther earth-potential related, electrically conductive foil layerclose to the surface and fully covering the circuit board without havingconductors formed therein and solely presenting a plurality of smallthrough-penetrating holes each having an adapted and dimensionedcross-sectional size which only slightly exceeds the cross-sectionalsize of an electric conductor, a pin or the like, drawn through thehole.

It will also be seen that a technical problem resides in providingconditions such that each of the electric conductors drawn through saidholes in the layer foil can be connected either directly or indirectlyat one end with one of the connecting conductors, connecting pads or pinconnectors of a discrete component, and at their other end connecteddirectly to a conductor in a conductor pattern formed in an electricallyconductive layer where said electrically conductive layer may be earthpotential related, with the exception of the conductors.

It will also be seen that a technical problem resides in realizing thesignificance of limiting the cross-sectional size of athrough-penetrating hole in the layer foil to a circular cross-sectionhaving a diameter smaller than 1.0 mm.

A particular technical problem is one of not only realizing but alsoobserving that said holes form a resonance circuit having a resonancefrequency, and that the size of said holes should be adapted so that theresonance frequency will occur first at above some 100's GHz, andtherewith adapted to a level which is far above the highest frequencyapplicable to the circuit board and its components.

It will also be seen that a technical problem resides in realizing thoseadvantages that are obtained when the conductor drawn through a hole iscomprised of a connection sleeve whose one end is able to coact with aconnection island or the like.

In respect of two electromagnetic interference screening layers of whichone forms said layer foil, a foil which solely includes smallconductor-adapted holes, and the other forms a foil in which electricalconnection islands are formed, said two foils being mutually isolatedelectrically by an insulating foil, a further technical problem is oneof realizing the significance of enabling said two electromagneticinterference-screening layers to be earth-potential related andpair-wise located on a respective side of a number of signal layers inthe form of metal foils.

It will also be seen that a technical problem is one of providingconditions, with the aid of simple means, which enable each of the twoelectromagnetic interference screening layer foils to be located on itsrespective side of a number of signal layers with intermediateelectrically insulating layers and with pad layers on at least one side.

Another technical problem in the present context is one of realizing thesignificance of electrically connecting said two layer foils one to theother and with two mutually opposing electrically conductive layers.

It will also be seen that a technical problem resides in thesignificance of mutually connecting said screening layer foils throughthe medium of through-plated edge-orientated holes and by spacing theholes at a small distance apart, for instance a spacing of less than 3mm, preferably about 1 mm, so as also to screen the edge surfaces of thecircuit board.

Another technical problem is one of realizing the significance ofmutually connecting said screen or shielding layer foils by copperplating.

Solution

With the intention of solving one or more of the aforesaid technicalproblems, the present invention takes as its starting point a circuitboard which is constructed of a plurality of electrically conductivelayers, conductor patterns formed in said layers, and intermediateelectrically insulating layers, and which further comprisessurface-related electrically conductive layers which screens againstelectromagnetic interference, wherein an outermost conductive layer isprovided with a number of coordinated connection islands or the like,where selected connection islands are adapted for electrical andmechanical coaction with connection pads or the like for connection of asurface-mounted discrete component.

In accordance with the present invention, it is proposed that a circuitboard of this kind is provided with a further electrically conductivelayer which is located close to and beneath said outermost conductivelayer and isolated therefrom by an insulating layer, and which iscomprised of layer foil which screens against electromagneticinterference, wherein said electrically conductive layer foil hasprovided therein a number of through-penetrating holes, each of whichhas an adapted and dimensioned cross-sectional size which slightlyexceeds the cross-sectional size of an electric conductor drawn throughsaid hole, and wherein one end of said conductor is connected, eitherdirectly or indirectly, to one of said connection conductors, connectionpads or the like, and at its other end is connected directly to aconductor in a conductor pattern formed in an inner, additionalelectrically conductive layer.

According to preferred embodiments that lie within the scope of theinventive concept, the through-penetrating holes have a diameter smallerthan 1.0 mm.

The conductor drawn through a hole may be comprised of a connectionsleeve whose one end coacts with a connection island.

According to one particular embodiment, two layer foils which screenagainst electromagnetic interference are each located on a respectiveside of a number of signal layers, wherein said two layer foils areconnected electrically together, either directly or indirectly, and toearth potential.

According to one embodiment, the screening layer foils are mutuallyconnected through the medium of through-plated, edge-orientated holes,where the distance between adjacent holes is smaller than 3 mm,preferably about 1 mm.

The screening layer foils are conveniently connected together by copperplating.

Advantages

Those advantages primarily afforded by an inventive circuit boardarrangement reside in the creation of conditions whereby effectivescreening of EMI radiation can be achieved in a circuit board comprisedof a plurality of electrically conductive layers and conductor patternsformed therein, and intermediate electrically insulating layers, withthe aid of a further electrically conductive layer foil which functionsas an electromagnetic interference screen, and therewith provideconditions for obtaining an effectively screening layer foil havingsmall holes adapted for through-passing conductors.

The holes formed in the layer foil may have a size which results in ahole-related resonance frequency that is many times greater, at leastone-hundred times greater, than the highest circuit-board relatedfrequency.

The main characteristic features of an inventive circuit boardarrangement are set forth in the characterizing clause of the followingclaim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of an inventive circuit board arrangement at presentpreferred will now be described in more detail with reference to theaccompanying drawings, in which

FIG. 1 is a perspective view of part of a circuit board with a discretecomponent mounted thereon;

FIG. 2 illustrates an underlying layer foil formed in accordance withthe invention and screening, essentially completely, againstelectromagnetic interference;

FIG. 3 is a view of a layer foil only partly screening againstelectromagnetic interference; and

FIG. 4 is a perspective, partially sectioned view of a circuit boardwith surface-mounted discrete components, wherein part of the layerfoils have been removed for the sake of clarification.

DESCRIPTION OF EMBODIMENTS AT PRESENT PREFERRED

FIG. 1 shows in perspective part of a circuit board 1 and a discretecomponent 2 mounted on the surface thereof.

Discrete components 2 of the kind intended here normally include aplurality of connection conductors or connection pins, although for thesake of simplicity only five connection conductors of a semi-conductorcomponent 2 are shown in FIG. 1. These connection conductors aredesignated 2a . . . 2d and each is connected to a respective connectionpad or connection island 3a . . . 3e, formed in an uppermost andoutermost pad layer 3.

The pad layer 3 is comprised of an electrically conductive foil and isearth-potential related with regard to those sections that do not formconnection pads.

Located beneath the pad layer 3 is an electrically insulating foil and afirst screening electrically conductive layer foil 4 in accordance withthe invention, said layer foil 4 being connectable to earth potential 5'by means of a conductor 5.

The illustrated circuit board also includes an electrically insulatedfoil and an earth-potential related second foil layer 6, which isfollowed by a plurality of electrically conductive layers havingconductor patterns formed therein, and intermediate electricallyinsulating layers coordinated in a region which has been shown generallyat 7, since these layers are not required in obtaining an understandingof the present invention.

The circuit board 1 may conveniently be provided with an additional padlayer 3', an additional electromagnetic interference screening foillayer 4', and an additional layer 6' which screens partially againstelectromagnetic interference on the opposite side of the circuit board.

The circuit board 1 thus includes a surface-related layer 6 whichscreens against electromagnetic interference, wherein an outermostconductive layer 3 is provided with a number of coordinated connectionislands 3a . . . 3e, where selected connection islands are adapted forelectrical and mechanical coaction with connection conductors 2a . . .2e for a discrete surface-mounted component 2, a so-called semiconductorcomponent.

The invention is based on the use of a further electrically conductivelayer 4 which is contiguous with the aforesaid outermost conductivelayer 3 and which is comprised of an outermost layer foil which totallyscreens against electromagnetic interference.

The electrically conductive layer foil includes a number ofthrough-penetrating holes 4a . . . 4e, each of which has across-sectional size which is slightly greater than the cross-sectionalsize of an electric conductor, a pin or the like, drawn through thehole.

One end of each such electric conductor is connected, either directly orindirectly, with one of said connection conductors 2a . . . 2e, whilethe other end is connected directly to a conductor (6a) in a conductorpattern formed in still another electrically conductive layer (6)coacting with the conductor pattern within the region 7.

The diameter of the through-penetrating holes 4a . . . 4e is smallerthan 1.0 mm and the holes are positioned in the layer foil 4 incoordination with the connection pads 3a . . . 3e.

According to one embodiment, the conductor drawn through a hole maycomprise a connection sleeve (40) whose one end coacts with one of saidconnection pads 3d.

It is particularly proposed in accordance with the invention that twolayer foils 4, 4' that screen against electromagnetic interference areeach disposed on a respective side of a number of intermediate signallayer foils within the region 7.

The two layer foils 4, 4' are mutually connected electrically.

Said two pairs of layer foils 4, 4' and 6, 6' are also connected toearth potential.

Each connecting sleeve is comprised of a plated copper sleeve.

The aforesaid screening layer foils 4, 4' and 6, 6' respectively aremutually connected via through-plated edge-orientated holes 8a . . . 8e,said holes having a spacing which is preferably smaller than 3 mm, andpreferably about 1 mm.

The aforesaid screening layer foils 4, 4' and 6, 6' may also be mutuallyconnected by copper plating 9.

The electrically conductive foil 6 is provided with conductor sections6a, 6b and 6c, by etching away a surface section 6'.

It will be understood that the conductor sections 6a, 6b and 6c may bein electrical coaction with the conductors 2a, 2b and 2c.

The section 6" that is not etched away functions as an electromagneticscreen through the shown earth potential connection.

The invention provides at least two closely-related electromagneticscreening layers on each side of the circuit board.

It will noted in particular that solely partially screening layers, apad layer 3, a totally screening layer 4 and a partially screening layer6 may be coordinated to achieve the desired screen function.

It will be understood that the invention is not restricted to theaforedescribed and illustrated exemplifying embodiment thereof, and thatmodifications can be made within the inventive scope defined in thefollowing Claims.

It will also be understood that the inventive concept can be applied toany type of discrete component connection, i.e. even connections thathave not been described or mentioned with respect to the illustratedembodiment.

We claim:
 1. A circuit board which comprises:a plurality of electricallyconductive layers, conductive patterns formed in said layers, andintermediate electrically insulating layers, a surface-related layerwhich screens against electromagnetic inference, wherein an outermostconductive layer is provided with a plurality of coordinated connectionislands, where selected connection islands are adapted for electricaland mechanical coaction with connection pads for a discretesurface-mounted component, wherein a further electrically conductivelayer located contiguous with and beneath said outermost conductivelayer and separated therefrom by an insulating layer is comprised of alayer foil which screens against electromagnetic interference; in thatsaid electrically conductive layer foil has formed therein a pluralityof through-penetrating holes, each of which is adapted and dimensionedwith a cross-sectional size which slightly exceeds the cross-sectionalsize of an electric conductor drawn through a respective hole, saidelectric conductor being connected at one end, either directly orindirectly, with one of said connection pads and connected at its otherend directly to a conductor in a conductor pattern formed in an inner,further electrically conductive layer wherein said layer foil includestwo layer foils which screen against electromagnetic interferencelocated on a respective side of a plurality of intermediate layers.
 2. Acircuit board according to claim 1, wherein the through-penetratingholes are selected to have a diameter smaller than 1.0 mm.
 3. A circuitboard according to claim 1, wherein the conductor drawn through saidhole is a connection sleeve whose one end coacts with one of saidconnection islands.
 4. A circuit board according to claim 1, whereinsaid two layer foils are electrically connected one to the other.
 5. Acircuit board according to claim 4, wherein said two layer foils areconnected to earth potential.
 6. A circuit board according to claim 3,wherein the connecting sleeve is a copper-plated sleeve.
 7. A circuitboard according to claim 1, wherein said screening layer foils aremutually connected through the medium of through-plated andedge-orientated holes.
 8. A circuit board according to claim 7, whereinthe spacing of the holes is selected to be less than 3 mm, preferablyabout 1 mm.
 9. A circuit board according to claim 1, wherein saidscreening layer foils are mutually connected by copper-plating.
 10. Acircuit board according to claim 2, wherein the conductor drawn throughsaid hole is a connection sleeve whose one end coacts with one of saidconnection islands.
 11. A circuit board according to claim 1, whereinsaid two layer foils are electrically connected one to the other.
 12. Acircuit board according to claim 1, wherein said screening layer foilsare mutually connected through the medium of through-plated andedge-orientated holes.
 13. A circuit board according to claim 1, whereinsaid screening layer foils are mutually connected by copper-plating.